It is well known in the vehicle fuel system art to maintain a specific vapor pressure head within the vapor space of a fuel tank during refueling to prevent over filling of the fuel tank. Vapor pressure increases due to ambient temperature change, or the heating of the fuel during operation of the vehicle, require controlled venting of vapor from the fuel tank.
A frequently employed device is the ball-type head valve connected between the fuel tank and a vapor trap such as a carbon canister. In this type of valve a weighted ball normally sits on a valve seat to limit vapor venting from the tank and thereby build up and maintain a specified head pressure. The ball is responsive to certain conditions, either excess vapor pressure in the fuel tank or vehicle movement, to unseat and vent vapor from the fuel tank to the vapor trap. The ball valve is often calibrated or balanced by spring structure.
U.S. Pat. No. 4,760,858 to Szlaga discloses a ball head valve contained within a cup-shaped retainer and normally resting on a valve seat to block vapor venting from the fuel tank. The ball is held very loosely by the cup-shaped retainer to permit substantial movement or rattling of the ball valve off the valve seat when the vehicle is in motion. When the vehicle is stationary, the ball valve opens only in response to excess vapor pressure in the fuel tank. When the vehicle is in motion, the ball continuously vibrates off the valve seat within the loosely-toleranced cup-shaped retainer to intermittently bleed vapor from the fuel tank and reduce the effective head pressure therein.
U.S. Pat. No. 4,666,058 to Harris discloses a ball valve contained within a cup-shaped retainer depending from the cap on the end of a vehicle filler pipe. The cup-shaped retainer has a plurality of vent holes formed about the periphery of the retainer above the normal resting position of the ball. A vent outlet is formed in the top of the retainer above the peripheral vent holes. When the vehicle is upright the valve is normally open, with the ball valve resting below the radial vents to permit fuel vapor to flow out the vent outlet at the top of the retainer. The location of the vent holes above the ball prevent its being prematurely closed by a vapor pressure from the fuel tank; i.e., the ball is only affected by liquid level or rollover orientation.
Numerous other patents disclose pressure- or movement-operated ball-type head valves: U.S. Pat. No. 1,893,942 to Jensen; U.S. Pat. No. 3,738,384 to Hall; U.S. Pat. No. 4,392,507 to Harris; U.S. Pat. No. 4,416,108 to Ghandhi; U.S. Pat. No. 4,457,325 to Green; U.S. Pat. No. 4,736,863 to Harris; and U.S. Pat. No. 4,779,755 to Harris.
The vapor trap carbon canisters in modern vehicle fuel systems frequently become partially saturated with fuel vapor and must be purged to the vehicle manifold to restore their effectiveness. The purged fuel vapor is burned off by the engine. Experience has shown that prior art head valves, including ball-type head valves, have an adverse effect on engine performance. Movement-operated, rattle-type valves are likewise unsuitable for use as a primary head valve. Prior art pressure-operated valves tend to blast open and snap closed in an abrupt fashion, resulting in either a sudden charge of fuel rich vapor to the manifold, or a sudden decrease in the vapor being supplied to the manifold. This sudden increase or decrease in supplied fuel vapor during the purge cycle causes undesirable engine roughness or "stumble".
However, ball head valves do possess the advantage of being mechanically simple and reliable. It is therefore desirable to improve the vapor flow characteristics of the ball-type pressure-responsive head valve to eliminate the purge cycle problems associated with them.